Higher coverage of unit tests

[alessandrodalbello]

To increase the testing coverage, strengthen the existing ones, and re-organize the packages inside the test folder to reflect the ones present in main. This will open to refactors and code adjustments headed to improve the performance and code readability. It will also make it easier for developers to spot possible defects in the current implementation.

Possibly, a coverage badge could be added to the README page and automatically updated during the Github workflow execution (e.g. the Jacoco badge generator).